Friction elements designed integrally with the pump wheel or turbine wheel normally have a low wall thickness and are therefore unfavorable for lying closely together. An axial pressure between the friction elements may therefore be uneven, whereby a local increase in pressure force can occur. In this region, the friction elements may become overheated, which can result in increased wear or permanent deformation of one of the friction coatings. Also, in this region, the fluid may be heavily thermally loaded, whereby it can also be damaged.